This invention relates generally to broadband communication systems, and more particularly the invention relates to a spectrally efficient broadband communications system using a media attachment unit, or modem, which emulates an attachment unit interface used on a baseband medium. In a particular application, the invention provides a carrier sense multiple access network with collision detection (CSMA/CD) meeting the IEEE 802.3 10Broad36 specification using 12 MHz transmission channels.
The most common type of local area network for transmitting data between computers is CSMA/CD, supra. This type of network is specified by Ethernet and IEEE 802.3, "CSMA/CD Access Method and Physical Layer Specifications". The network consists of a number of nodes (computers) connected to a common bus. Each node has a unique address and can transmit packets of data to any other node on the bus. A requirement of this type of network is that all nodes have equal access to the network. This is accomplished by collision detection and enforcement. If two nodes attempt to transmit at the same time, one or both will detect a collision and transmit an alarm. All nodes will cease transmission and delay retransmission for a random period of time.
The transmission medium for Ethernet and IEEE 802.3 10Base5 and 10Base2 is a coaxial cable onto which Manchester encoded baseband data is transmitted. Unfortunately this medium provides a very limited diameter (maximum distance between two remote nodes) due to attenuation from the cable. The maximum specified diameter for a baseband network is 500 meters. In addition, baseband transmission is not spectrally efficient, with the entire bandwidth of the cable occupied by one signal.
IEEE 802.3 10Broad36 was an attempt to address the deficiencies of baseband transmission by modulating the signal onto a broadband medium. The diameter was expanded to 3750 meters, and the entire signal was limited to two 18 MHz bands, one upstream, one downstream, so that the cable could be used for other applications. The 18 MHz bandwidth included the modulated 10 Mbit/s data, as well as bandwidth reserved for out of band signalling for collision enforcement. Unfortunately, implementation of 10Broad36 is very expensive, requiring special circuitry for generating and detecting the out of band collision enforcement signal, and requiring a total of 36 MHz of bandwidth for signal and collision enforcement.
The present invention provides a spectrally efficient and inexpensive means of transmitting data over a broadband network. The use of "holes" and phase modulation of the data for collision detection and signalling, eliminates the need for out of band signalling, reducing the expense of the hardware, and moving collision detection to the headend. This nearly doubles the maximum network diameter for full collision enforcement.